1. Field of the Invention
The invention relates to a process for the production of a catalyst body for the catalytic treatment of gas, in particular for the catalytic purification of exhaust gas from an internal combustion engine. Such catalyst bodies intended for incorporation into a housing of a catalytic converter are frequently also referred to as substrates. The internal combustion engine may, for example, belong to an automobile or other motor vehicle or may be stationary.
2. Description of the Prior Art
A catalytic converter disclosed in U.S. Pat. No. 5,187,142 has a catalyst body with rectangular corrugated sheet metal members which are stacked one on top of the other and each of which has waves arranged in a herringbone pattern. Between successive groups of such corrugated sheet metal members are arranged retainer sheet metal members, each of which has a rectangular, corrugated main segment and, at two edges of this which face away from one another, an angled flap. In the production of such a catalyst body, untreated corrugated sheet metal members and retainer sheet metal members are first produced and are stacked one on top of the other. Thereafter, the sheet metal members are connected to one another at their points of contact by brazing or discharge welding and the overlapping flaps are welded to one another and possibly also to the corrugated sheet metal members to form a packet of sheet metal members firmly connected to one another, the flaps together forming inner and outer surfaces with steps. Coatings containing catalytically active material are then applied to the one or more connected sheet metal members. The sheet metal members then together bound passages for the exhaust gas.
In this production process, the production and assembly of the sheet metal members requires a relatively large number of operations. Furthermore, it is difficult and expensive to apply more or less uniform coatings to the entire surfaces of the corrugated sheet metal members and of the main segments of the retainer sheet metal members after assembly of a packet of sheet metal members. In order for the stated surfaces to be more or less completely covered by coatings, the corrugations must be made relatively high. In addition, the corrugations of adjacent sheet metal members must intersect one another so that each pair of adjacent sheet metal members bounds only a single passage which is divided only at the points of contact of the sheet metal members. The catalyst body therefore has only a small number of passages per unit cross-sectional area in a cross-section transverse to the general direction of flow of the exhaust gas, those sections of the surfaces of a passage which are opposite one another generally being large distances apart. For these reasons, the catalyst body produces only a small purification effect per unit volume of the catalyst body and must therefore be relatively large to permit sufficient exhaust gas purification. The known catalyst body and a catalytic converter equipped with such a catalyst body therefore have the disadvantages that they require a large amount of space and are correspondingly heavy. Furthermore, during a cold start, the large mass of the known catalyst body increases the time required for the catalyst body to heat up to a temperature advantageous for effective exhaust gas purification. In addition, the catalyst bodies contain expensive materials, in particular usually at least one noble metal present in the coatings, so that the large mass of a known catalyst body also increases the production costs of the catalytic converter.
In a process disclosed in Japanese Patent Application 6,254,409 for the production of a catalyst body, flat and corrugated sheet metal members are inserted into two sleeves and fastened therein. The two sleeves are then welded to one another. Coatings containing a catalytically active material are then applied to the sheet metal members. The corrugations of the corrugated sheet metal members of catalyst bodies produced in this manner must likewise have large wave heights to enable coatings to be applied to the sheet metal members after the latter have been assembled. The production process disclosed in Japanese Patent Application 6,254,409 and the catalyst body produced by said process therefore have disadvantages which are substantially similar to those of the production process and the catalyst body according to U.S. Pat. No. 5,187,142.
European Patent Applications 0,676,534 and 0,676,535 which corresponds to U.S. Pat. Nos. 5,645,803 and 5,593,645 respectively, disclose catalyst bodies, each of which has rectangular, flat and corrugated sheet metal members and spacer members arranged at two edges of the sheet metal members, which edges face away from one another. For the production of such a catalyst body, sheet metal members are produced according to European Patent Application 0,676,534 and have a corrugated main segment provided with coatings and flat, untreated edge segments, to which spacer members are provisionally fastened by spot welding. Furthermore, flat sheet metal members with a main section having coatings and untreated edge segments are produced. Thereafter, the sheet metal members are stacked one on top of the other and are welded along their stated edges to the spacer members and to one another. However, this production process is rather expensive.
In produced catalytic converters of this type, the lengths of the catalyst body and sheet metal members are at least 100 mm and the widths of the spacer members are about 5 mm. The spacer members and the untreated edge segments of the sheet metal members, which segments are covered by said spacer members, have a relatively large mass which, during a cold start, increases the time required for the catalyst body to heat up to the optimal operating temperature. Continuous tests with catalytic converters of the type described have moreover shown that, under high stresses, in particular in the case of relatively long catalyst bodies, the result may be permanent deformations of sheet metal members, which impair the action of the catalyst body.